1. Field of the Invention
The present invention relates to an improved fluidized bed boiler used for a combustion of a fuel such as pulverized or crushed coal, by which air pollution due to flue gases discharged from the boiler is reduced. In particular, it relates to a two-bed type boiler comprising an upstream combustion zone forming a fluidized bed of a particulate heat medium and a downstream desulfurizing zone forming a fluidized bed of a particulate sulfur acceptor.
2. Description of the Related Art
As is well known, a fluidized bed boiler comprises a combustion zone defined by a vertically extending vessel of a cylindrical or rectangular shape with a perforated gas distribution plate arranged horizontally and a heat exchanger for producing steam over the plate. A fluidized bed of a particulate heat and fluidizing medium lodged on the plate is formed in an upper part of the zone downstream of the plate and combustion particles such as pulverized coal are supplied downstream of the plate as fuel. Air is fed into a lower part of the zone upstream of the plate as a fluidizing and oxidizing gas and passes upwards through the horizontal plate. The combustion particles in a fluidized state react with the air to generate a combustion heat.
This heat energy is directly or indirectly imparted to the heat exchanger located within the fluidized bed. In the indirect heat transfer, the heat medium particles in the fluidized state are heated and come into contact with the surfaces of the heat exchanger to impart sensitive heat thereto.
In such a conventional fluidized bed boiler, sand or limestone is used. When the limestone is used as the heat and fluidizing medium and also sulfur accepter, it is removed from the boiler after a certain period of the boiler operation and is dumped after the heat energy thereof is recovered.
Solid particles including unburnt coal, spent lime and ash, which are entrained in the flue gases produced in the boiler, are separated from the flue gases by a cyclone. The separated solid particles are then returned to the original boiler or sent to a second combustion or a re-combustion zone provided to form another fluidized bed of the separated solid particle, air is fed as a fluidizing and oxidizing gas, and a fresh combustion material such as coal or heavy oil is fed as a supplemental fuel for re-combustion.
In general, a solid particle fuel such as coal contains non-combusted ash particles, and with such a solid fuel, larger size ash particles are accumulated in the fluidized bed of the boiler during a long operation period of the boiler. These accumulated ash particles are manually periodically discharged from the fluidized bed and dumped after being subjected to a heat recovery process by a heat exchanger provided in the boiler system.
In a prior art fluidized bed boiler of this type, flue gases produced from the boiler are sent to a sulfur removal unit through a conduit communicating the boiler and the removal unit before being discharged out of the boiler system, to prevent air pollution by the sulfur content of the flue gases. The sulfur removal unit removes the sulfur content by techniques such as wet scrubbing with water or other liquids such as glycols or amine and absorption and/or reaction of sulfur oxides on or with solid acceptors such as limestone or the like.
Also, in the prior art, the sulfur removal unit is incorporated with the fluidized bed boiler by a mechanical separator such as a cyclone for separating from the flue gases solid particles such as ash entrained therein.
A conventional fluidized bed boiler is known wherein limestone is used in place of sand as a heat, sulfur acception and fluidization medium, to enable the sulfur content of the flue gases to be removed during the combustion process per se. In this case, however, preferably the boiler is also equipped with a sulfur removal unit in practice, because the sulfur content of the flue gases is not sufficiently removed during the combustion and desulfurizing process by the limestone and thus the flue gases cannot be discharged from the system due to the danger of air pollution. This is due to the practical difficulties encountered when attempting to harmonize an optimum temperature condition for a combustion of the coal with that for a desulfurization of the flue gases, which condition must be applied in the same fluidized bed of limestone, with the result that an efficient desulfurization or sulfur removal is not obtained in the combustion and desulfurizing process.
Recently, an improved fluidized bed boiler was developed, which is a vertically arranged two bed type consisting of an upstream and lower zone forming a fluidized bed of sand for only combustion and a downstream and upper zone forming another fluidized bed of limestone for only desulfurization or sulfur removal. The two zones are separated by a gas distribution plate through which flue gases produced in the lower fluidized combustion bed are passed into the upper fluidized desulfurization bed as fluidizing gases to be desulfurized. The temperature conditions for combustion and desulfurization in the separate fluidized beds are regulated to optimum levels for the respective functions, compared with the conventional one bed type boiler using limestone.
Such a two bed type boiler is more advantageous than the one bed type boiler using limestone and the bed type boiler using sand but provided with the additional sulfur removal unit in reducing the sulfur content from the flue gases to be discharged, while ensuring a high heat efficiency of the entire boiler system.
However, when a low grade and high ash content coal like "culm" is burnt, the inventors have recognized that the upper fluidized bed zone with the limestone does not fully exert the expected inherent ability for desulfurization, for the following reasons. One, that the gas distribution plate separating the two beds is partially clogged, mainly with ash particles entrained in the flue gases, so that the flow per se of the flue gases from the lower combustion bed zone is obstructed; and two, that CaSO.sub.4 produced by desulfurization of the sulfur content of the flue gases with the limestone lowers the melting point of the ash, with the result that an increased amount of the ash is deposited on the surface of the particulate limestone, leading to an obstruction of the desulfurization process.